Sanitary land filling accounted for disposal of about 69-73% of all MSW in the United States in 1988 (EPA, 1990). In 1990, about 6,000 landfills were operating. The EPA reported that 45% of those should reach capacity by 1991, leaving 3,300 in operation after that date. About one-half of the 6,000 operating landfills are very small, as shown in Table 6.1. These smaller landfills will account for most of the anticipated closures (FR, l991h). The trend is thus toward operating fewer, but larger landfills (NSWMA, undated).
More recent estimates do not support the expectation that the number of operating landfills will rapidly decrease. Various estimates of operating landfills published in 1990 and 1991 range between 5,300 and 7,300. The data from different sources were quite inconsistent and uncertain (Repa and Sheets, 1992).
Gas Recovery Status
In 1991, approximately 157 U.S. landfills operated or planned to operate landfill-gas-to-energy facilities. Two-thirds use the recovered gas to produce electricity (Berenyi and Gould, l991b). The EPA estimates that 87 large landfills built between 1992 and 1997 will include facilities for the recovery and combustion of landfill gas (FR, l991p).
Table 6.1 SIZE DISTRIBUTION OF MSW LANDFILLS(a) Landfill Size Percentage of Percentage of Total (Tons per Day) Total Landfills Waste Handled 1-17.5 51 2 17.6-50 17 4 51-125 13 9 126-275 7 11 276-563 5 16 564-1,125 3 19 >1,125 3 40 Source: Federal Register, 1991n (1986 data) (a) Numbers may not add because of rounding
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Landfills require energy for construction, compacting, spreading daily landfill cover, collecting and treating leachate, and similar activities. The operation of a landfill uses 0.09-0.28 million Btu per ton of MSW (SRI estimate based on fragmentary data; see Exhibit V).
To complete the energy balance, the amount of energy produced (methane) must be determined, and the fraction that can be collected and used must be estimated. Unlike combustion, in which energy is released very quickly, the conversion of MSW to methane can require a long time. For analysis, a period must also be chosen for the recovery of energy from the landfill. The following discussion presents the data and assumptions used in the energy production portion of the analysis.
Although only 157 of the nation's approximately 6,000 operating landfills are operating or plan to operate landfill gas-to-energy plants, the energy and emissions data in this report are based on landfill with gas recovery. The largest landfills (about 200 have a capacity of more than 1,000 tons per day) are more likely to include the energy recovery facilities, and those landfills now receive more than 40% of all MSW landfilled in the United States. In comparison with facilities that either collect landfill gas and flare it or allow the gas to escape into the atmosphere, landfill gas-to-energy operations reduce environmental releases of methane while providing an energy benefit.
Modern landfills with gas recovery facilities can ultimately produce 1 to 1.8 standard cubic feet of methane per pound of dry waste (Augenstein and Pacey, 1991). Variations in the amount of methane generation in a single landfill and from landfill to landfill are at least 100% (Augenstein and Pacey, 1991). The rate at which methane is generated varies even more widely. The methane production rate is often measured as the time a landfill takes to produce one-half of the total quantity of the gas that it will eventually release-a period that can range from 2 to more than 25 years, with a range of 5-15 years being typical (Augenstein and Pacey, 1991). The higher production rates (i.e., the shorter time periods in the range) require extremely wet conditions.
Length of Time for Energy Production
Proposed EPA regulations call for a minimum of 15 years of active methane extraction from large landfills. Over a 10- to 20-year period, which is the approximate maximum life of a commercial gas extraction project (CEC, 1991)(3), a landfill can produce about 1.3 to 2.4 million Btu per ton of wet MSW(2); a well-maintained extraction operation can recover 85% of that amount for fuel use (Augenstein and Pacey, 1991). The energy thus recovered amounts to 18% to 24% of the energy that could be recovered through combustion of the same MSW.(4) For the balance calculated here, 100% gas recovery has been used. The Electric Power Research Institute (EPRI) estimates the fuel value of MSW at 4,500 Btu per pound at 30% moisture (EPRI, 1989).
Energy Potential of All Landfills
According to EPA estimates, all U.S. landfills combined release 12 million tons per year of methane (FR, l991b). If all that methane could be captured and used to generate electricity, the landfills would provide 5% of the natural gas used in the United States to supply electricity (calculated from FR, 1991b, and DOE, 1991).
Addition of water to a landfill has been shown to approximately double the amount of methane generated-from 20%-25% of the theoretical maximum anaerobic conversion of MSW to methane (Augenstein and Pacey, 1991; Augenstein, 1992) to about 50% (Morelli, 1990). Added water also increases the rate at which the methane is released (Bogner, 1992). "Stimulation" of landfills (i.e., addition of optimal quantities of water and nutrients) could potentially double methane production over the rate for unstimulated landfills (Bogner, 1992). At least five research projects on enhancing landfill gas production were under way in 1991; the EPA allowed those landfills to use leachate recycling (Thornloe, 1991).
Net Energy Balance
The net result over 20 years of active methane extraction and use is the difference between the production-of energy (1.3 million to 2.4 million Btu per ton of MSW) and the energy required for landfill-associated operations (0.090 million to 0.280 million Btu per ton of MSW). The range then is 1 million to 2.3 million excess Btu recoverable per ton of MSW placed in a landfill. As noted, the energy is based on landfill operations only; the entire energy balance, consisting of collection, land filling, and compaction, is discussed under "Integrated Strategy Example."
Given the extreme variability of a relatively uncontrolled process like gas generation in a landfill(5), selecting a single value to represent generation or emissions for all landfills is useful only to provide a benchmark for comparisons with other technologies. To provide a sense of the magnitude of these releases and the net energy they could produce, this report presents estimates based on "reasonable" values cited in the literature. Other estimates, perhaps based on conditions at particular sites, can be substituted in the computerized data base to determine the effect of variations on the net energy balance for a given facility.
Figure 6.3 shows U.S. landfill tipping fees by state. The New England, Great Lakes, and Atlantic and Pacific coastal states have the highest landfill tipping fees, and the Plains and southern states have the lowest fees. By state, landfill costs range from highs of $50-$150 per ton in New Jersey to lows of $3-$5 per ton in South Dakota The higher costs in some states result from the scarcity of suitable landfill sites, dense population concentrations in metropolitan areas, tighter state environmental regulations, fees, and higher transportation and labor costs. Cost variations from state to state and region to region can strongly influence the degree of desirability of choosing alternative strategies for MSW management.
RANGE OF 1991 U.S. LANDFILL TIPPING FEES BY STATE
(Dollars per Ton of Municipal Solid Waste(a))
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Because construction often continues throughout the life of the landfill, rather than being completed at the beginning of operations, capital costs are often mixed with operating costs. Capital and operating costs of landfills can be estimated by using models; however, such models are valid only for a particular region, and even then they are quite uncertain. For example, a model developed for estimating costs for landfills in Michigan estimated a construction cost of $25.5 million for a 20-year, 1,000 ton per day landfill. The estimate for the total project cost, $125 million, was conservative and could underestimate actual costs by as much as 100%, according to the model developers (Walsh, 1990).
The few studies that were found indicate wide variability in landfill costs as a result of local conditions. These costs, separated into cost elements, are shown in Figure 6.4.
Data on ash disposal costs were found for only seven RDF plants (see Exhibit I). The costs range from $3 to $57 per ton of ash. For two new plants in the Northeast, the cost averages $26 per ton of the ash. That cost is lower than the cost for MSW disposal, and the ash amounts to 17% by weight of the original MSW.
LANDFILL CAPITAL AND O&M COSTS(a)
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